Field of the Invention
The invention relates to a holographic projection screen and its method of production.
A projection system consists of an emitting image of small size, which is magnified by suitable optics and projected onto a screen. The emitting image is generally an active or non-active transmitting element illuminated by a collimated light source.
When the projection is so-called front projection, the observer is placed on the same side as the projector in relation to the screen, which then reflects the image towards the observer.
When the projector illuminates the rear of the screen in relation to the observer, it is a back-projecting screen.
Given the low brightness of the images coming from the projector, it is preferable for the screen to be directional and to have gain so that a large part of the light emitted by the screen reaches the observer's eye.
Moreover, in the case of front-projection screens, it may be advantageous for reasons of convenience to be able to position the projector, not in the center in relation to the projection screen, but to the sides or above or below. This type of projector operation is usually obtained by off-centering the emitting image in relation to the optical axis of the projection system.
However, in order for the light scattered by the screen to reach the observer, it would be necessary in this case for the screen with gain to possess a luminance indicatrix off-axis in relation to the direction of the reflected mid-ray. Thus, as is shown in FIG. 1, an incident beam F1 (projection beam) is reflected off the screen SC, in a conventional manner, along a direction F2. In order for it to reach the observer, it is necessary for it to be directed along a lobe direction F3.
Such a screen, being on principle outside the laws of geometrical reflection, cannot be produced in a conventional structure. However, it is known how to produce, in conventional optics, screens which, in transmission, bring about these scattering functions with "off-axis" gain. These are complex structures of the Fresnel-lens type. As a consequence, standard front-projection systems use screens either with very low gain (movie theater), or of the Lambertian type.
Moreover, the low contrast of the projected image, once the ambient illumination increases, should be noted as a major drawback of front-projection devices. This is connected with the very nature of the screen, of the white-surface type, which rescatters a large part of the incident light irrespective of its source.
Devices of the "back-projection" type, the scattering screen of which possesses a "black matrix" which enables the contrast of the image to be substantially increased for the same ambient illumination, are thus generally preferred.
In order to illustrate the state of the art, the following document describing various projection devices is cited:
M. KAWASHIMA et al, "Display and projection devices for HDTV", IEEE Transactions on Consumer Electronics, Vol. 34, No. 1, February 1988, 100-110.